Recently, in the field of battery cans, it is desired that their body weight is reduced, that the processes for producing them are simplified, that the costs of raw materials for them and also the production costs are reduced, and that the walls of their side body parts are thinned to thereby increase their capacity for fillers, for which so-called 2-piece battery cans where the cathode terminal side and the side body part are integrated have been developed (see Japanese Patent Publication No. 7-99686), and have already been put into practical use. The process of producing such 2-piece battery cans comprises a step of blanking a steel sheet having a thickness of from 0.4 to 0.5 mm or so to give steel discs and thereafter deep-drawing each steel disc into a cylindrical part, followed by a step of ironing said cylindrical part with a plurality of ironing dies thereby to produce a side body part of the can having a prolonged length while thinning the wall of said side body part. The wall of the side body part of the can as produced through said process shall finally have a reduced thickness of not larger than 0.15 mm.
Therefore, the steel sheet for such 2-piece battery cans is required to have good formability in ironing or the like, and also to have high mechanical strength and good anti-secondary work embrittlement while it is in a side body part of the can. In addition, since the terminals and other parts of such 2-piece battery cans are easily corroded, the steel sheet for said cans is required to have good corrosion resistance, for which said steel sheet is generally coated with an Ni plate to ensure its corrosion resistance.
The following techniques have heretofore been proposed for steel sheets for 2-piece battery cans.
(1) A technique of preventing micro-cracks from being formed on the surfaces of 2-piece battery cans during draw-and-ironing of a steel sheet to give said cans in consideration of the battery characteristics and the corrosion resistance of 2-piece battery cans, the absence of said micro-cracks being helpful in ensuring good battery characteristics of the cans (see Japanese Patent Application Laid-Open No. 5-21044). PA1 (2) A technique of ensuring a uniform temperature and a uniform drawing ratio of a steel sheet for 2-piece battery cans during the steps of hot-rolling and cold-rolling said steel sheet in order to attenuate the in-plane anisotropy of the resulting rolled steel sheet and to enhance the uniformity thereof in the direction of its coiling width, thereby controlling the final steel sheet to have a defined r value and a defined .DELTA.r value (see Japanese Patent Application Laid-Open No. 6-344003). PA1 (3) A technique of producing a steel sheet for 2-piece battery cans through continuous annealing, in which the carbon content of said steel sheet is defined to be not larger than 0.009% by weight (substantially from 0.002 to 0.003% by weight) (see Japanese Patent Application Laid-Open No. 6-346150). PA1 (4) A technique of defining the surface roughness of a steel sheet for 2-piece battery cans to thereby prolong the life of the mold to be used for draw-and-ironing molding of said steel sheet (see Japanese Patent Application Laid-Open No. 6-346282). PA1 (5) A technique of controlling the film thickness and the morphology of the Ni plate layer to be formed on the surface of a steel sheet for 2-piece battery cans, thereby ensuring the corrosion resistance of said steel sheet. (see Japanese Patent Application Laid-Open No. 6-346284). PA1 (1) A steel sheet for 2-piece battery cans with excellent formability, which contains S and O (provided that O is the total oxygen content) while satisfying the conditions that S is from 0.005 to 0.015 wt. %, that O is from 0 to 0.0025 wt. % and that [S/10+O].ltoreq.0.0035 wt. %. PA1 (2) A steel sheet for 2-piece battery cans with excellent formability, which contains S and O (provided that O is the total oxygen content) while satisfying the conditions that S is from 0.005 to 0.012 wt. %, that O is from 0 to 0.0025 wt. % and that [S/10+O].ltoreq.0.0030 wt. %. PA1 (3) A steel sheet for 2-piece battery cans with excellent formability, anti-secondary work embrittlement and corrosion resistance, which contains B of from 0.0005 to 0.0015 wt. % while satisfying the condition that 0.2.ltoreq.B/N.ltoreq.1.0, and contains S and O (provided that O is the total oxygen content) while satisfying the conditions that S is from 0.005 to 0.015 wt. %, that O is from 0 to 0.0025 wt. % and that [S/10+O].ltoreq.0.0035 wt. %. PA1 (4) The steel sheet for 2-piece battery cans with excellent formability, anti-secondary work embrittlement and corrosion resistance of the above-mentioned (3), which contains S and O (provided that O is the total oxygen content) while satisfying the conditions that S is from 0.005 to 0.012 wt. %, that O is from 0 to 0.0025 wt. % and that [S/10+O].ltoreq.0.0030 wt. %. PA1 (5) The steel sheet for 2-piece battery cans with excellent formability, anti-secondary work embrittlement and corrosion resistance of the above-mentioned (3) or (4), which contains one or both of Cr of being from 0.03 to 0.10 wt. % and Ni of being from 0.01 to 0.10 wt. %, the total of the two being 0.10 wt. % or smaller. PA1 (6) A steel sheet for 2-piece battery cans with excellent formability and corrosion resistance, which contains one or both of Cr of being from 0.03 to 0.10 wt. % and Ni of being from 0.01 to 0.10 wt. %, the total of the two being 0.10 wt. % or smaller, and contains S and O (provided that O is the total oxygen content) while satisfying the conditions that S is from 0.005 to 0.015 wt. %, that O is from 0 to 0.0025 wt. % and that [S/10+O].ltoreq.0.0035 wt. %. PA1 (7) The steel sheet for 2-piece battery cans with excellent formability and corrosion resistance of the above-mentioned (6), which contains S and O (provided that O is the total oxygen content) while satisfying the conditions that S is from 0.005 to 0.012 wt. %, that O is from 0 to 0.0025 wt. % and that [S/10+O].ltoreq.0.0030 wt. %. PA1 (8) The steel sheet for 2-piece battery cans with excel lent formability and corrosion resistance of the above-mentioned (6) or (7), which contains one or both of Cr of being from 0.05 to 0.10 wt. % and Ni of being from 0.03 to 0.10 wt. %, the total of the two being 0.10 wt. % or smaller. PA1 (9) The steel sheet for 2-piece battery cans of the above-mentioned (1), (2), (3), (4), (5), (6), (7) and (8), of which the both surfaces have at least a Ni plate layer or an Fe--Ni alloy plate layer.
These prior art techniques are to define the steel sheet for 2-piece battery cans and the method for producing said steel sheet as disclosed in the above-mentioned Japanese Patent Publication No. 7-99686. However, none of these suggests any explicit means of defining the optimum components of constituting the steel sheet for 2-piece battery cans in order that the steel sheet may have good DI moldability, that the steel sheet may have high mechanical strength and good anti-secondary work embrittlement while being in cylindrical forms, that the steel sheet may have good Ni-platability with homogeneous diffusion of Ni into the surface of said steel sheet being coated, and that the Ni-plated steel sheet may have good corrosion resistance. Thus, said prior art techniques are not ones for producing steel sheet substrates capable of being formed into side body parts of cans having thin walls. In particular, with the expected requirement for much thinned walls of side body parts of 2-piece battery cans in further, it is anticipated that the steel sheet for said cans shall be controlled much more severely not only for preventing the steel sheet from being cracked or wrinkled during forming it into cans but also for reducing inevitable impurities that are inevitably introduced into the steel sheet during its production. Given the situation, none of the above-mentioned prior art techniques could satisfy the requirements for the steel sheet for 2-piece battery cans.
Accordingly, one object of the present invention is to solve the problems with the above-mentioned prior art techniques, and to provide a steel sheet for 2-piece battery cans that can be well formed into 2-piece battery cans through DI molding with excellent formability while satisfying the requirement for thinning the walls of the side body parts of the cans formed.
The other object of the present invention is to provide a steel sheet for 2-piece battery cans, which has excellent formability in DI molding and, after having been formed into cans, still has high mechanical strength and excellent anti-secondary work embrittlement with excellent corrosion resistance, and, in particular, which can be well subjected to DI molding into cylindrical side body parts at an ironing reduction ratio of 70% or higher to have much thinned walls without being cracked at the flanges and without being cracked by ironing and, after having been formed into cans, still has high mechanical strength and excellent anti-secondary work embrittlement with excellent corrosion resistance at the embossed terminals.